Pulmonary transplantation remains the only available treatment for numerous patients, who suffer from end-stage lung disease. However, outcomes after lung transplantation continue to be substantially worse than those after transplantation of other organs. Current immunosuppressive strategies for lung transplant patients are based on protocols that have been successfully used for recipients of other organs. However, such approaches do not account for fundamental differences between immune responses to lung grafts and other commonly transplanted solid organs. Utilizing new mouse models of lung transplantation we have uncovered that tolerance to pulmonary grafts is regulated locally through interactions of immune cells within lymphoid aggregates that are newly formed in the graft. We have reported that disruption of immunoregulatory circuits in tolerant grafts results in the local activation of graft-infiltrating lymphocytes, which triggers rejection. This application proposes to study mechanisms how tolerance is maintained at the level of the graft microenvironment after pulmonary transplantation and will allow for the development of new therapies for lung transplant recipients.